Fixed #19779 -- Checked contrib.sites presence in RedirectFallbackMiddleware

Thanks Aymeric Augustin for the report and directions for the patch.

django/contrib/redirects/middleware.py

 from __future__ import unicode_literals
 
+from django.conf import settings
 from django.contrib.redirects.models import Redirect
 from django.contrib.sites.models import get_current_site
+from django.core.exceptions import ImproperlyConfigured
 from django import http
-from django.conf import settings
+
 
 class RedirectFallbackMiddleware(object):
+    def __init__(self):
+        if 'django.contrib.sites' not in settings.INSTALLED_APPS:
+            raise ImproperlyConfigured(
+                "You cannot use RedirectFallbackMiddleware when "
+                "django.contrib.sites is not installed."
+            )
+
     def process_response(self, request, response):
         if response.status_code != 404:
             return response # No need to check for a redirect for non-404 responses.

django/contrib/redirects/tests.py

 from django.conf import settings
 from django.contrib.sites.models import Site
+from django.core.exceptions import ImproperlyConfigured
 from django.test import TestCase
 from django.test.utils import override_settings
 from django.utils import six
 
+from .middleware import RedirectFallbackMiddleware
 from .models import Redirect
 
 
@@ -52,3 +54,10 @@ class RedirectTests(TestCase):
             site=self.site, old_path='/initial', new_path='')
         response = self.client.get('/initial')
         self.assertEqual(response.status_code, 410)
+
+    @override_settings(
+        INSTALLED_APPS=[app for app in settings.INSTALLED_APPS
+                        if app != 'django.contrib.sites'])
+    def test_sites_not_installed(self):
+        with self.assertRaises(ImproperlyConfigured):
+            RedirectFallbackMiddleware()

docs/ref/contrib/redirects.txt

@@ -13,11 +13,12 @@ Installation
 
 To install the redirects app, follow these steps:
 
-1. Add ``'django.contrib.redirects'`` to your :setting:`INSTALLED_APPS`
-   setting.
-2. Add ``'django.contrib.redirects.middleware.RedirectFallbackMiddleware'``
+1. Ensure that the ``django.contrib.sites`` framework
+   :ref:`is installed <enabling-the-sites-framework>`.
+2. Add ``'django.contrib.redirects'`` to your :setting:`INSTALLED_APPS` setting.
+3. Add ``'django.contrib.redirects.middleware.RedirectFallbackMiddleware'``
    to your :setting:`MIDDLEWARE_CLASSES` setting.
-3. Run the command :djadmin:`manage.py syncdb <syncdb>`.
+4. Run the command :djadmin:`manage.py syncdb <syncdb>`.
 
 How it works
 ============

docs/ref/contrib/sites.txt

@@ -246,6 +246,7 @@ To do this, you can use the sites framework. A simple example::
     >>> 'http://%s%s' % (Site.objects.get_current().domain, obj.get_absolute_url())
     'http://example.com/mymodel/objects/3/'
 
+.. _enabling-the-sites-framework:
 
 Enabling the sites framework
 ============================

docs/releases/1.5.txt

@@ -653,6 +653,12 @@ Miscellaneous
   Attempting to load it with ``{% load adminmedia %}`` will fail. If your
   templates still contain that line you must remove it.
 
+* Because of an implementation oversight, it was possible to use
+  :doc:`django.contrib.redirects </ref/contrib/redirects>` without enabling
+  :doc:`django.contrib.sites </ref/contrib/sites>`. This isn't allowed any
+  longer. If you're using ``django.contrib.redirects``, make sure
+  :setting:``INSTALLED_APPS`` contains ``django.contrib.sites``.
+
 Features deprecated in 1.5
 ==========================